Yes, I've looked this up but, find too much mumble/jumble. Can someone explain in more simple terms of what exhaust scavenging is/dose...without writing a book? Or is there too much to detail in order to explain briefly?

That's an apt description, but in the case of exhaust, it's frequency dependent and usually does not start until north of 3,000 RPM ... It is tune-able like a Pipe Organ tubes. Hence the search for the right length and size of collector tube and extension. It does not generate HP per se, but helps achieve more air flow and that helps with the HP.

There is a YT vid on Engine Masters where they test various exhausts and you can see the results compared

Formerly in marine engineering. In an earlier life I owned my own petroleum tank truck, and hauled for the majors and independent's.